Very dumb. Apparently it's a catchprase among kids to ask "You mad bro?" when someone is clearly angry, in order to infuriate the target still further. I assume this like saying "Don't be irrational" to someone who's being irrational -- fuel for the fire.

Some kids held up a sign at a football game reading "You mad bro?" and... they are being attacked for racism, because obviously "bro" is a racial slur, and also because no adult bothered to find out what the catchphrase actually means.